If you are responding to the first post, a quick reply works great. There is a Post Reply button at the top and bottom of the thread - it looks like this:

Just click it and type your response. Then click Submit Reply to post it.

To Quote a post so everyone will know what you are responding to, use the Quote button under the post you would like to respond to. It is the button on the left pictured below:

Then type your response under the text that automatically appears and click Submit Reply to post it.

Finally, if there are several posts you'd like to respond to, you can Multi-Quote them.

To do this, you'll want to click the Multi-Quote button under every post you want to quote EXCEPT the last one. The Multi-Quote button has a little green circle with a plus mark in it. It is the center button pictured below.

When you click it, the green circle will change to red, and the plus mark will become a minus sign.

For the last post you want to quote, use the main Quote button (far left button).

The window that loads will now have the posts quoted. Be sure to type after the end of the quote as shown below:

If you look for the [/quote] that indicates the end of the quoted post, and always type after that, your reply will never get 'stuck' in the middle of the quote